High-voltage bipolar-CMOS-DMOS integrated circuit devices and modular methods of forming the same

Abstract

All low-temperature processes are used to fabricate a variety of semiconductor devices in a substrate the does not include an epitaxial layer. The devices include a non-isolated lateral DMOS, a non-isolated extended drain or drifted MOS device, a lateral trench DMOS, an isolated lateral DMOS, JFET and depletion-mode devices, and P-N diode clamps and rectifiers and junction terminations. Since the processes eliminate the need for high temperature processing and employ “as-implanted” dopant profiles, they constitute a modular architecture which allows devices to be added or omitted to the IC without the necessity of altering the processes used to produce the remaining devices.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is a divisional of application Ser. No. 11 / 443,745, filed May 31, 2006, which is incorporated herein by reference in its entirety. [0002] This application is related to application Ser. No. 10 / 262,567, filed Sep. 29, 2002, now U.S. Pat. No. 6,855,985, which is incorporated herein by reference in its entirety.BACKGROUND OF THE INVENTION [0003] This invention relates to semiconductor chip fabrication and in particular to methods of fabricating, integrating and electrically isolating high-voltage and low-voltage bipolar, CMOS and DMOS transistors and passive components in a semiconductor chip monolithically without the need for high temperature fabrication processing steps. [0004] In the fabrication of semiconductor integrated circuit (IC) chips, it is frequently necessary to electrically isolate devices that are formed on the surface of the chip, especially when these components operate at different voltages. Such complet...

AI Technical Summary

Benefits of technology

[0055] In accordance with this invention, a series of processes are used to integrate high-voltage and DMOS transistors with fully-isolated floating pockets of low-voltage CMOS, bipolar transistors, diodes, and passive circuit components. The processes eliminate the need for high temperature processing and epitaxy and employ “as-implanted” dopant profiles—ones where the final dopant profiles remain substantially unaltered from their original implanted profiles by any subsequent wafer processing steps. Together, the processes form a modular architecture which allows devices to be added or omitted to the IC and the corresponding process steps added to or removed from the integrated flow without the necessity of altering the processes used to produce the other devices on the IC.

[0063] A process of fabricating a depletion-mode MOS device may include forming a gate over a semiconductor surface, implanting a drift region self-aligned to the gate, and implanting source and drain regions. The process may also include using a chained implant to form a deep drain region. In an alternative embodiment the drift region is implanted prior to the formation of the gate and is therefore not self-aligned to the gate. In yet another embodiment a deep conformal drift region is implanted prior to the formation of the gate. Each of the foregoing embodiments may be modified to include a subsurface shield to reduce the onset on NPN parasitic bipolar conduction and to suppress snapback effects. The depletion-mode device may also be fabricated in a fully isolated form with a deep isolation layer overlapped by an annular sidewall isolation wells that may also function as a deep drain.

Problems solved by technology

Conventional CMOS fabricated in P-type substrate material does not facilitate complete isolation of its devices since every P-type well forming the body (back-gate) of NMOS transistors is shorted to the substrate potential, typically the most negative on-chip potential.

High temperature processing causes a redistribution of dopant atoms in the substrate and epitaxial layers, causing unwanted tradeoffs and compromises in the manufacturing of dissimilar devices fabricated using one common process.

Moreover, the high-temperature diffusions and epitaxy employed in epi-JI processes are generally incompatible with the large wafer diameters and advanced low-temperature processing equipment common in submicron CMOS fabs.

The voltage differential between source and body causes a number of problems.

A high threshold in turn increases on-resistance while lowering saturation current, adversely impacting switch performance.

Finally, without a low resistance body contact, snapback breakdown can occur easily.

Integration of a source body short into a large area NMOS, while common in discrete power devices, requires isolation of the P-type body from the P-type substrate in integrated form, something conventional CMOS cannot offer.

Processes offering such isolation are complex to manufacture, often requiring high temperature fabrication steps.

The resulting body effect causes the threshold of NMOS 22 to increase substantially, making it difficult to provide adequate gate drive to achieve a low on-resistance without damaging the thin gate oxide of NMOS 22.

Integration of a source body short into a large area NMOS, while common in discrete power devices, requires isolation of the P-type body from the P-type substrate in integrated form, something conventional CMOS cannot offer.

Processes offering such isolation are complex to manufacture, often requiring high temperature fabrication steps.

Without isolation, such a device cannot be integrated monolithically with other components or circuitry.

A disadvantage of AC switch 45 is its high specific on-resistance, i.e. a large RDSA, since the two series connected transistors exhibit additive resistances.

Aside from the need to integrate NMOS devices with isolated source body shorts, another limitation of conventional CMOS is its inability to prevent undesirable snapback breakdown effects in MOSFET operation, particularly in NMOS transistors.

Negative resistance is especially problematic in power electronic circuitry, giving rise to excess currents, oscillations and instability, electrical noise, localized heating, thermal runaway and even device destruction.

Because of surface charge and other unavoidable surface effects, however, the equipotential lines do not spread themselves uniformly, but instead “bunch up” near the gate edge resulting an a locally higher electric field at the end of the drift region.

Even worse, the high electric field is physically located near a region of high current density.

Since impact ionization creates electron-hole pairs, two undesirable effects result.

First the electrons are accelerated to high energies relative to the crystal, i.e. they become energetically “hot”, and may get swept into the gate oxide damaging the dielectric.

The result is another cause of negative resistance since more impact ionization causes a high local field and contributes to even more current.

Moreover, the two negative resistance effects can occur simultaneously, interacting in a complex and even unpredictable way.

If the drain is driven into avalanche at high currents while sustaining voltage BVDSS, it may suddenly collapse back to BVCER, causing the current to increase and destroying the device.

If the NMOS is operating as a current source or switching from on to off, the onset of snapback may be exacerbated by increased substrate leakage due to impact ionization.

The other effect is that the parasitic NPN gain is too great since there is not enough base charge in the lightly doped substrate.

One obvious way to reduce the NPN transistor's adverse influence is to increase substrate doping, but unfortunately doing so also increases the electric field at the drain leading to even more impact ionization and substrate current.

The extra N− drift doping may be added to optimize the tradeoff between breakdown and resistance but remains limited by impact ionization effects where the gate juxtaposes the drift region.

The diffusion, taking anywhere from 7 to 24 hours, requires high temperatures over 1050° C. and typically 1100° C. or higher, a process incompatible with many modern low-temperature fabrication facilities and large wafer diameters.

The lateral implant method is complex and undesirable for manufacturing since the implant must be performed four times to cover all four gate orientations on a wafer.

Rotating the wafer during implantation makes uniform implantation difficult.

Unfortunately such a profile means the surface electric field is higher than in the bulk away from the surface, not ideal for manufacturing robust avalanche-rugged devices.

The high temperature diffusions involved in DMOS body fabrication are further complicated by the steps needed to achieve full electrical isolation of circuitry using epitaxial junction isolation.

As described previously, the problem with conventional epitaxial and high-temperature processes and manufacturing methods used to fabricate, isolate, and integrate high-voltage devices is that each high temperature process causes dopant redistribution affecting every high-voltage and low-voltage device.

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